9.5mm calculates as 0.374 inch, so I would vote for 3/8". Since the lathe is of mature years, the thread form will probably be Whitworth form. The pitch will decide whether it is BSW or BSF.

Hope that what follows is not "Teaching Granny to suck eggs"

With regard to the Tailstock Centre, a thread will not provide a repeatable centre, unless there is a register onto which it will locate. A taper, such as a 1MT would be far better) You can make a centre by taking a suitable piece of steel, putting in the Headstock (Firmly and accurately located0, setting the Toolpost over by 30 degrees, and turning up a centre. Whilst you are about it, make another for the Headstock, against the time when you need one.

If there is an obvious taper (chamfer) at the entrance to the tailstock bore, you could use that as the location, screwing the centre hard against it for a location, (But you need to have turned the centre with a similar chamfer at the headstock.  To ensure that the taper/chamfer at the Headstock end is truly on centre, you can either lightly trim it (if there is one) or make a "collet" for the Headstock. Drill and tap a piece of steel in the Headstock, and turn a taper / chamfer without moving the workiece.  You now have a location which is on the centreline of your lathe, that you can now use to turn the centre for the Tailstock.

For a Tailstock Drill Chuck, make up another arbor for the Tailstock, but with the correct thread, or taper, for the drill chuck. Chucks for Pistol Drills used to use 3/8 – 24 UNF thread, whilst the later ones (usually reversible drills) use 1/2 – 20 UNF thread. Chucks intended for use in lathes or Milling Machines usually have a Jacobs taper, (which one will depend on the chuck, and its size), which you will need to turn yourself, so you will need to experiment a little and test with a felt tip marking pen, or Micrometer brand blue. When you have produced a taper which is a good match for the female taper in the chuck, you clean everything, and fit the chuck onto the taper with a little endwise force, and you should then be ready to go!

Howard

Edited By Howard Lewis on 10/08/2018 19:30:16

Barry,

Where are you? In UK? Whereabouts?

Possibly someone near would be prepared to assist you.

Howard

Barry, you have a PM.

Metal is like woodturning, but different in various ways, and should result in more precise end products.

We all learn something every day, and this is your opportunity, so grasp it with both hands!

As long as you take care, a huge new world will open up to you, and there are LOTS of folk on here with experience to offer advice and help.

Howard